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The Weilberg is of particular importance because of the quarry, which has been closed since 1940 and which provides an insight into the rock structure of the former Siebengebirge volcanic field. About 30 million years ago, volcanic activity began with a huge ash shower. Magma penetrated the tuff and burned it in a red contact zone; the tuff was arched and moved along faults. Finally, lava rose again. A young basalt intersperses the older one as a narrow sill and widens out in the soft tuff. The surface of the volcano is no longer preserved; long-term erosion carried away a large part of the tuff and basalt. The Rhine flowed over this plateau in the Pleistocene (Ice Age). Basalt was mined from the Weilberg quarry in the 19th and early 20th centuries. Magma penetrated the tuff and burned it into a red-colored contact zone; the tuff was arched and displaced along faults. Finally, lava rose again. A young basalt penetrates the older one as a narrow dike and widens in the soft tuff. The surface of the volcano is no longer preserved; long-term erosion has removed a large part of the tuff and basalt. The Rhine flowed over this plateau in the Pleistocene (Ice Age). Basalt was mined in the Weilberg quarry in the 19th and early 20th centuries